The present invention pertains to apparatus employed in the dynamic (i.e., during engine operation) adjustment of valve-timing in internal combustion engines as a means of optimizing engine performance, including power output, torque, and fuel efficiency.
Internal combustion, reciprocating piston engines, such as the conventional four-cycle (i.e., intake/compression/combustion/exhaust), single overhead camshaft engine 10 shown in simplified cross-section in FIG. 1, utilize one or more intake valves 11 to allow air or a mixture of air and fuel into the cylinder 20 for combustion, as well as one or more exhaust valves 12 to allow combustion gases to exit the cylinder 20 following combustion. The operation of these valves 11, 12 is sequenced in order to repeatedly charge the cylinder 20 with fuel and air either ahead of (in the case of four-cycle engines) or during (in the case of two-cycle engines) the piston's 25 compression stroke, and to repeatedly permit the discharge of exhaust gasses during the exhaust cycle. More particularly, the operational timing of these valves 11, 12 is controlled by a camshaft 30 rotatably connected (including, for example, a sprocket, chain, belt, etc.) via a geared linkage (not shown) to a rotating crankshaft 35 which supports and moves each piston 25 within its associated cylinder 20.
Still referring to FIG. 1, each of the intake 11 and exhaust 12 valves is biased to a closed position, as shown, by a spring 13 or other biasing means. Each of the intake 11 and exhaust 12 valves is further mounted on (or, alternatively, provided in contact with via, for instance, a lifter/push-rod linkage) a pivotable rocker arm 14 or 15, respectively, which rocker arms are positioned to selectively contact one of several corresponding cam lobes 31 axially disposed along the length of the camshaft 30. During rotational movement of the camshaft 30, a specific one of the cam lobes 31 will engage one of the intake 11 or exhaust 12 valve rocker arms 14 or 15, respectively, causing temporary pivoting movement of the rocker arm and, correspondingly, linear movement of the associated valve against the bias of spring 13 to its open position. Upon movement of the cam lobe 31 out of engagement with a valve 11 or 12, that valve is urged back to the closed position thereof by the biasing force of spring 13.
As those skilled in the art will appreciate, the timing or angular position of the camshaft 30 relative to the crankshaft 35 is critical in effecting engine performance. Moreover, such timing is not ideally constant through all engine speeds. Rather, it is preferable, for optimizing engine performance, that operation of the intake and exhaust valves be advanced or retarded in response to various engine operating conditions, including variations in torque, temperature, the fuel/air mixtures, engine speed, etc. Thus, a fixed camshaft—that is, a camshaft with an unchanging angular position relative to the angular position of the crankshaft—at best provides optimum engine performance only in a narrow range of engine operation.
To address this problem various means have been proposed, the most commonplace of which are apparatus for dynamically varying the angular position of the camshaft relative to the crankshaft to thus alter valve operation timing as appropriate to the engine's operating condition at a given time. The structure of such apparatus, also known as cam phasing devices or, more commonly, simply as cam-phasers, is exemplified in FIG. 2, adapted from the disclosure of U.S. Pat. No. 5,588,404, assigned to General Motors Corporation, and which disclosure is incorporated herein by reference in its entirety. In general, such cam phasers comprise a first rotatable element 50 mounted to the end of a camshaft 60 for synchronous rotational movement therewith. The first element 50 includes helical splines 51 on its outer surface. A second rotatable element 52 surrounds the first element 50 concentrically and has a drive member 53, such as a wheel, pulley, or sprocket, driven by the engine crankshaft (not shown). On an inner surface, the second element 52 is also provided with helical splines 54 arranged oppositely from the splines 51. A piston 55 is positioned between the first 50 and second 52 elements, the piston having helical splines on both inner and outer surfaces thereof, respectively, which splines mesh with one or the other of the splines 51, 54. Through axial movement of the piston 55, accomplished by controlled hydraulic pressure, the several splined surfaces cooperatively interengage to cause counter-rotation of the first 50 and second 52 elements relative to each other, thus changing the angular position of the camshaft 60 relative to the engine crankshaft.
Conventional cam phasers such as described are characterized by a number of drawbacks, including their relatively large dimensions, which necessitate larger engine compartments that translate to higher production costs. Conventional cam phasers also tend to have a relatively high mass, which adds to the rotational mass of the engine. Moreover, this mass is disposed outside of the bearing envelope of the camshaft, which disposition equates to additional stress on the camshaft as well as the mounting bearings the for. Finally, conventional cam phasers are characterized by a complex construction comprising numerous interrelated, individual components. This complexity increases manufacture and assembly costs, and further reduces the operating life of the apparatus.
It would, accordingly, be desirable to provide a cam phaser that overcomes the drawbacks associated with conventional cam phasers.